So-called two-component system heat-sensitive recording materials utilizing the coloration reaction between a colorless or slightly colored electron-donating dye precursor and an electron-accepting compound are disclosed, e.g., in Japanese Patent Publication Nos. 14039/70 (corresponding to U.S. Pat. No. 3,539,375 and British Pat. No. 1,135,540) and 4160/68, and Japanese Patent Application (OPI) No. 190886/84 (corresponding to U.S. Pat. No. 3,202,510) (the term "OPI" as used herein refers to a "published unexamined Japanese patent application").
Such two-component system heat-sensitive recording materials have the advantages that: (1) they utilize a primary coloration, and, therefore, do not require development; (2) they possess paper quality approximating that of ordinary paper; (3) they can be handled with ease; (4) they provide high density coloration; (5) heat-sensitive recording materials providing various coloration hues can be easily prepared; and the like, thus having great merits for use and being most popularly utilized as heat-sensitive recording materials.
Particularly in recent years, they have been remarkably utilized in the field of labels such as POS (point of sales) as well as in the fields of facsimile, recorders, and printers.
On the other hand, since these heat-sensitive recording materials utilize primary coloration, they have the defect that the reaction between a dye precursor and a color-developing compound can take place when they come into contact with a solvent or the like as well as when heat is applied thereto.
This is attributable to the fact that all of these heat-sensitive recording materials are made of organic substances and have therefore such a large solubility in a solvent that the reaction takes place in the solvent. Therefore, when in contact with a writing material such as an aqueous ink pen, an oily ink pen, a fluorescent ink pen, etc., a diazo developer, an adhesive or the like, white background of the heat-sensitive recording material can form color, or formed letters can disappear, and thus the commercial value of the material can be seriously spoiled.
In order to overcome this defect, it has been attempted, for example, to provide on a heat-sensitive color-forming layer a solvent resistant protective layer, as disclosed, e.g., in Japanese Patent Publication No. 27880/69 (corresponding to U.S. Pat. No. 3,445,216 and British Pat. No. 1,099,880), Japanese Patent Application (OPI) Nos. 30437/73, 31958/73, etc.
However, such techniques have the defects that resistance against various chemicals, oils, and water is still insufficient, that sensitivity is reduced, anti-sticking properties are insufficient, or blocking takes place, that production steps of such recording materials are complicated, and that such recording materials become costly, thus being industrially still unsatisfactory from an overall viewpoint.
In addition, Japanese Patent Application (OPI) No. 188392/82 describes a heat-sensitive recording material which has a protective layer containing a resin and a hardener. However, experiments of the inventors have revealed that the material shows still insufficient resistance when dipped in water.
Upon hardening of polyvinyl alcohol binder, widely known inorganic substances such as metal salts harden the binder so fast that they are industrially difficult to put into practice.
Further, it is difficult with hardeners as methylolmelamine, epoxy compounds, etc., to provide satisfactory water resistance, chemical resistance, and oil resistance.